<< "Support for macOS 12 (Monterey) will be ending soon." >>
The latest version of MacOS that is supported on your now your ten-year old Mac is MacOS 12 Monterey.
Your Mac can continue to do everything it could do on the day you took it out of the box, plus a lot more because Apple has provided multiple version upgrades to MacOS, Free.
--------
That alarm is a statement from Quicken that they are not willing to test their software compatibility with older macOS.
Their message should be read as:
QUICKEN is no longer willing to put in the work to Test and provide Support for newer versions of Quicken for MacOS 12 (Monterey).